Ink jet printing is a conventional technique by which printing is normally accomplished without contact between the printing apparatus and the substrate on which the desired print characters are deposited. Such printing is accomplished by ejecting ink from an ink jet printhead of the printing apparatus via numerous methods which employ, for example, pressurized nozzles, electrostatic fields, piezo-electric elements and/or heaters for vapor phase bubble formation.
The ink compositions used in ink jet printing typically employ, for example, water, colorants and low molecular weight water-miscible solvents. The colorants which may be employed include dyes or pigments. Pigments often are preferred and they are generally characterized as colorants that are not soluble in the desired liquid vehicle of an ink composition. In order to prepare a pigment-based ink, therefore, pigment agglomerates typically are dispersed in a dispersant so that the resulting colorant concentrate can be added to the liquid vehicle to produce an ink.
When preparing the colorant concentrate, pigment agglomerates are typically reduced in size and dispersed in a dispersant. This often is achieved by mixing the pigments with the dispersant, followed by a grinding step. The grinding step may take place in commercially available ink preparation mills like ball and pebble mills. When grinding in these types of mills, conventional grinding media such as glass, stainless steel and zirconium oxide often are employed in order to enhance the grinding process. Unfortunately, however, conventional grinding media enhance the grinding process while simultaneously decreasing the purity of the resulting colorant concentrate, or the subsequent ink composition prepared therefrom. Such a decrease in purity can result in ink discolorations, ink pH alterations and malfunction of printing apparatuses.
It is of increasing interest to prepare colorant concentrates that are substantially pure. This invention, therefore, is directed to a novel method for grinding colorants with, for example, a small grain size grinding medium, to unexpectedly produce a substantially pure colorant concentrate. "Substantially pure", as used herein, is defined to mean less than 0.04%, and preferably, less than about 0.03%, and most preferably, less than about 0.025% impurities by weight, based on total weight of the colorant concentrate, as determined by Inductively Coupled Plasma-Atomic Emission Spectroscopy. Such impurities originate from the grinding media and/or the mill employed.